The present invention relates generally to gas turbine engines, and, more specifically, to compressors therein.
A turbofan gas turbine engine includes in serial flow communication a fan, multistage axial compressor, combustor, high pressure turbine (HPT), and low pressure turbine (LPT). Air is pressurized in the compressor and mixed with fuel and ignited in the combustor for generating hot combustion gases which flow downstream through the HPT and LPT which extract energy therefrom. The HPT powers the compressor through a rotor shaft therebetween and the LPT powers the fan through another rotor shaft therebetween.
In a multistage axial compressor many rows or stages of compressor rotor blades are used for increasing air pressure as the air flows downstream through the compressor from stage to stage. The compressor blades are mounted to the perimeter of corresponding rotor disks which are suitably axially joined together to form a common rotor assembly mounted at opposite ends in suitable bearings.
A typical rotor disk includes axial dovetail slots in the perimeter thereof in which axial dovetails of the corresponding compressor blades are retained. Or, the disk perimeter may include a circumferential dovetail slot for receiving circumferential-entry blade dovetails. In both cases, centrifugal loads generated by the blades during rotary operation are carried through the disk radially inwardly into a thinner web and in turn into a thicker hub having substantial hoop strength for accommodating the blade loads.
The disks are separately manufactured from the individual blades, all with precise manufacturing tolerances for maximizing the uniformity of the blades and disks circumferentially around an axial centerline axis. Since the compressor rotor operates at high rotational speed, it must be manufactured with minimal radial eccentricity from the centerline axis and with minimal mass unbalance therearound. In this way, minimal unbalance and vibration therefrom may be obtained for obtaining smooth performance of the engine.
However, bladed disks require that the supporting disks have sufficient size and corresponding mass to withstand the centrifugal loads and corresponding stresses. The disks, therefore, typically have enlarged hubs with minimal diameter bores suspended from the disk perimeter by thin webs.
Compressor rotor design may be enhanced by replacing one or more of the bladed disks with unitary or one-piece blisks in which the blades or airfoils are integrally formed with the perimeter of the supporting disk without dovetails. The improved mechanical strength of the unitary airfoils and supporting disk in the blisk permit a substantial reduction in size of the disk and associated weight. Weight reduction is a significant design objective in producing light weight turbofan engines for powering aircraft in flight.
Blisks, however, introduce a corresponding problem in repair thereof which may be required after extended usage in service. Foreign object damage of compressor airfoils requires either repair thereof or replacement as warranted by the degree of damage. For a bladed disk configuration the individual blades may be readily removed from their corresponding dovetail slots in the disks, and remotely repaired or simply substituted by a replacement blade.
Since blisk airfoils are integrally formed with the supporting disk the airfoils are not individually removable from the blisk, which therefore requires that the entire blisk be removed from the compressor for repair or replacement thereof. Blisk removal requires compressor teardown which is both time consuming and expensive during a maintenance repair overhaul.
A typical compressor rotor for a multistage axial compressor includes several rotor components having one or more rotor stages which are axially joined together at corresponding radial flanges. There are several radial flanges between the opposite ends of the collective rotor which have corresponding bearing seats which are supported in bearings during operation. The several rotor components must therefore be assembled for minimizing eccentricity relative to the engine axial centerline axis, and balanced as an assembly for reducing unbalance.
Each radial flange has a flat annular face including a plurality of circumferentially spaced apart axial holes through which mounting fasteners in the form of bolts are used and retained by corresponding nuts for joining together the adjoining rotor components. The opposite ends of each rotor component are subject to manufacturing variation in dimensions which typically result in small amounts of relative eccentricity and tilt or non-perpendicularity of the end planes. When the rotor components are assemble together, eccentricity and tilt of the individual components stack together. This requires suitable pre-inspection of the components and deliberate indexing or clocking thereof for minimizing the relative eccentricity at the supporting bearing planes of the rotor.
Furthermore, the assembled rotor then undergoes a balancing procedure for minimizing undesirable unbalance thereof. The so assembled and balanced rotor may then be installed in its corresponding compressor casing during assembly of the compressor and engine.
Although an individual blade may be removed from a bladed disk without requiring compressor rotor teardown, a blisk airfoil cannot. Instead, the entire blisk must be removed and requires engine and compressor teardown, and corresponding reassembly of the compressor rotor and balancing thereof in the elaborate process described above.
Small compressor blisks up to about 30 cm in outer diameter have been commercially used in this and other countries for many years. Such small blisks are used in relatively small engines. One or more blisks may be used in a compressor rotor and typically include curvic couplings on the opposite axial faces of the hub which mate with corresponding couplings on adjoining rotor components. Alternatively, the blisks may include integral annular mounting arms with corresponding radial flanges for being attached to adjacent rotor components.
In either embodiment, the blisks form an integral component of the collective compressor rotor and are part of its structural integrity, and necessarily carry torque loads in series between adjoining rotor components. The collective rotor also provides a continuous loadpath between the mounting bearings for carrying various rotor loads to the bearings. The blisks, therefore, are not removable without compressor rotor teardown.
In view of the substantial weight reduction attributable to blisks over bladed disks, substantially larger blisks up to about 90 cm, for example, are being developed for substantially larger turbofan gas turbine engines in the 70,000 pound thrust class, and higher. These large blisks are considerably expensive. The corresponding compressor and engine teardown required for repairing such a large blisk in a large compressor has a correspondingly large maintenance expense associated therewith.
Accordingly, it is desired to provide an improved compressor rotor having one or more blisks therein which may be repaired without requiring complete compressor rotor teardown during a maintenance overhaul.
A compressor rotor includes coaxially adjoining disks having corresponding rows of rotor blades. A forward shaft is affixed to a forward disk and includes an annular seat for being mounted in a bearing to support the rotor at its forward end. A blisk coaxially surrounds the shaft axially between the seat and forward disk, and is removably fixedly joined piggyback to the shaft. In a repair method, the blisk may be removed from the compressor rotor without requiring teardown of the compressor rotor itself.